2.F : Airplane Weight and Balance

Weight has a significant impact on flight. A heavier gross weight will result in higher takeoff speeds, longer takeoff runs, reduced rate and angle of climb, lower maximum altitude, shorter range, reduced cruise speed, reduced maneuverability, higher stall speed, higher approach and landing speeds, a longer landing roll, and excessive weight on the nose/tail wheel. The reduced climb and cruise performance can result in higher engine temps and increased fuel use.

High weight can have impacts on the structure, as well, and in extreme cases can result in airframe failure. Even when not catastrophic overloading will increase incremental wear over time possibly resulting in required repairs. As long as gross weights remain in limits aircraft performance will remain in limits, and the airframe will remain safe within the published load factor limits.

Overloading can also result in control and stability problems. While weight distribution has the most significant effect, overloading itself can reduce the stability of an aircraft. If the weight is forward-loaded the aircraft will handle in a "heavier" manner, reducing maneuverability, and generally requiring more total lift from the wings due to the increased downforce generated (and needed) by the tail. If the weight is rearward-loaded the aft CG will make the aircraft handle "lighter", will increase cruise speed, will require less total lift be generated due to the reduced downforce on the tail, and reduces the stall speed. HOWEVER, if stalled the aircraft will be slower to recover.

The reason stall speed increases with increased weight is that as the weight goes up, more lift is required at any given airspeed to lift the higher weight. More lift is generated by angle of attack (AOA), and therefore at a higher weight the AOA is greater for any given airspeed than if the weight was lower. Since stall is entirely a function of AOA, then at any given airspeed the aircraft is closer to the crititcal AOA with higher weight then with lower weight, thus it is already closer to a stall.

This is also related to the fact that as weight increases, Va decreases, which for many is counter-intuitive. But the reason is simple. A stall serves as a "circuit breaker" if a gust or turbulence attempts to accelerate the aircraft. Because an aircraft is flying at a higher AOA (at a given speed) with higher weight, it means that it is already closer to the critical AOA and, thus, if a gust accelerates the aircraft upward, further increasing the AOA, then the aircraft will stall sooner due to the higher weight. This will relieve the load factor on the aircraft preventing structural damage.

Unbalanced lateral loads can cause problems as well, and can be caused by fuel imbalance, passenger loading, or baggage/cargo loading. Compensating with trim can help, but this puts the aircraft out of an optimally streamlined condition and will reduce performance.

Generally an aircraft becomes less controllable as the CG moves aft. This is due to the fact that the elevator has a shorter effective arm. Stall recovery is more difficult due to the aircraft’s reduced tendency to pitch down, and if the CG moves too far after recovery from a stall (or spin) becomes impossible. The aft CG is also more efficient, thus causing the aircraft to fly faster at a given power setting.

As the CG moves forward the plane becomes more nose-heavy, and the elevator may run out of the ability to compensate for that condition. This can be pronounced during landing and low speed operations. The forward CG is also less efficient, thus causing the aircraft to fly slower at a given power setting.

The pilot is responsible for management of weight and balance, and the flight manual should be used with approved methods and charts to determine a safe W&B. Never exceed the manufacturer’s limitations. The approved methods can vary and may include the use of CG calculations, CG graphs, and CG tables. Some manufacturers chart and graph the loading in terms of moment rather than CG, and in each case it is important to know what units are being used as you calculate the weight and balance.

As noted above, the CG is the total moment divided by the total weight. Start with the basic empty weight and then list all the items to be loaded. This includes people, objects/baggage/cargo, and fuel, noting the weights of every item. First insure that the total weight is less than the max gross weight for the aircraft. If the weight is too great, something must go.

Then calculate the moments of each item included, using the methods documented in the aircraft flight manual (AFM or POH). This document should include the basic empty weight and moment for this specific aircraft.

Then calculate the CG for this specific aircraft with this specific loading. Add up the moments to get the total moment, and the weights to get the total weight, and then divide the moment by the weight to get the CG.

If there is a change in loading, that change in CG can be calculated with a simple equation. In the following equation M^1 and W^1 are the original moment and weight.

Any weight causes a positive moment change, and weight removed causes a negative moment change. Weight shifted rearward causes a positive moment change and weight shifted forward is a negative moment change. If no weight is added, but is only moved, that only causes a change in the moment.